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Mass-To-Light Ratios for Elliptical Galaxies

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Mass-To-Light Ratios for Elliptical Galaxies Mass-to-light ratios for Elliptical galaxies Derek Land Supervisor: Professor R.H. Sanders January 28, 2008 Abstract With Newtonian dynamics, the masses of elliptical galaxies do not explain the masses derived from the luminosity of the galaxy itself. In this research, different methods to determine the masses of galaxies are compared to mass estimates of the same objects with modified Newtonian dynamics. We conclude that the modified Newtonian dynamics mass estimates agree well with stellar population models and mass estimates using lensing data. A problem for X-ray mass measurements is the assumption of hydrostatic equilibrium. This assumption might not be valid, which might explain the sometimes extremely high mass-to-light ratios for these objects. Contents 1 Introduction 3 1.1 Goal . 3 2 Background 4 2.1 Dark Matter . 4 2.1.1 Baryonic dark matter . 4 2.1.2 Non-baryonic dark matter . 5 2.2 Modified Newtonian Dynamics . 7 2.3 Newtonian virial theorem . 7 2.4 Fundamental plane . 8 2.5 Elliptical galaxies and the MOND Fundamental Plane relation . 9 3 Modelling of stellar kinematics 11 3.1 Comparing mass-to-light ratios . 12 4 Lensing 14 4.1 The `more' fundamental plane . 16 4.2 MOND and the `more' fundamental plane . 16 4.3 Mass-to-light ratios . 18 4.4 Conclusion . 18 5 Hot Gas 20 5.1 Mass from hot gas . 20 5.2 X-ray observations . 21 5.3 Hydrostatic equilibrium . 22 5.4 Conclusions . 23 6 Overall conclusions 24 6.1 Future work . 24 2 1 Introduction In 1937 Fritz Zwicky published a paper (Zwicky, 1937) in which he applied the Virial Theorem in order to calculate the dynamical mass of the Coma Clus- ter. After comparing the dynamical mass and the luminosity of the cluster he astonishingly found that the mass inferred from the luminosity of the cluster was about 400 times less then its dynamical mass. The gravitational attraction of the visible mass could not explain the line of sight velocities of the cluster. To explain the differences between the dynamical mass and the luminosity, he concluded that there must be some kind of non-visible form of matter, i.e. `dark matter'. In the 1970s, sensitive spectrographs made it possible to measure the velocity curve of edge-on spiral galaxies. Rogstad and Shostak (1972) showed that flat rotation curves exist up to 50kpc from the core of luminous spiral galaxies. The difference between the luminous mass and virial mass observed in spiral galaxies turned out to be part of a bigger missing mass problem. From dwarf spheroidal galaxies up to super clusters of galaxies, all pressure supported systems seem to be missing mass. 1.1 Goal It is well known that spiral galaxies have a missing mass problem, for elliptical galaxies the situation is less clear. In order to quantify and identify the missing mass problem, the mass of elliptical galaxies needs to be measured. In this report three different methods will be used: modelling of stellar kinematics, gravitational lenses and hot gas in order to estimate the mass, and thus the missing mass of a galaxy. In section 3 the modelling of stellar kinematics is used to determine mass-to-light ratios for a set of 25 elliptical galaxies from their observed velocity dispersion and effective radius. In section 4 data from the Sloan Lens ACS Survey is applied to the fundamental plane theorem to obtain mass estimates without the dependency on the luminous matter. And in section 5 X-ray emitting gas observed by the Chandra and XMM-Newton X-ray satellite observatories are used to construct the total mass of a galaxy from the hot gas distribution. In section 6 the results from the three different mass estimates are combined and compared to other estimates, in order to attempt to answer the question: can Modified Newtonian Dynamics explain the missing mass problem without dark matter in elliptical galaxies? 3 2 Background 2.1 Dark Matter With Newtonian dynamics, extra mass is required to explain the observed differ- ences between the dynamical mass and the luminosity of (clusters of) galaxies. The only possibilities explaining the missing mass problem are unknown par- ticles which do not or only weakly interact with baryonic matter, or theories which deviate from the known theory of gravity. 2.1.1 Baryonic dark matter Baryonic dark matter is ordinary matter, like protons and neutrons, except baryonic dark matter does not emit electromagnetic radiation. It is not clear why baryonic dark matter does not emit electromagnetic radiation. There are different possibilities in what kind of form dark matter can exist. A limited review is given below: The Initial Mass Function (IMF) is our estimate of the number of objects formed with a certain mass. Often the Salpeter IMF (Salpeter, 1955) is used, this is a power law dn m−x (1) d ln m ≈ where n is the number density of objects with mass m. For the Salpeter IMF the exponent is x = 1:35. Integrating from a certain minimum mass till infinity gives the total mass. But this total mass depends on the minimum mass used in the integration. If for the Salpeter IMF the minimum mass is changed by a factor of 7.2 the total mass is enlarged by a factor of 2, reducing the required amounts of dark matter. Changing the minimum mass of the IMF is constraint by the number of low mass objects which can exist. Low mass objects have a lower limit below which it is not possible to collapse. This limit is the Jeans mass which depends on the maximum scale, density and temperature for which a perturbation can be stable. If a cloud of gas is less massive then the Jeans mass, the cloud will not collapse. Forming low mass stars (M∗ M from a single cloud of gas is not possible because the mass of the cloudwill stay below the Jeans mass. To be able to create low mass objects, there must be some larger cloud of gas which fragments into smaller pieces. When a cloud collapses the internal energy rises, heating the gas and halting the collapse. The cooling has to be fast; if the cooling is slow and the cloud is pressure dominated the cloud will collapse without forming smaller substructures. In order to explain the missing mass problem with small, collapsed cloud fragments, a lot of these objects need to be around. The pro- cess of forming small, Jupiter sized, objects is difficult and not well understood, making it unlikely that all the dark matter can be explained by these objects. Small bodies such as comets, dust grains or asteroids, which are held together due to molecular forces are an unlikely source of dark matter. Although they are too small and faint to detect at any astronomical distance beyond the solar 4 system, they can not represent much of the missing mass. They mostly consist of carbon, silicon and oxygen, which are not common compared to hydrogen in the universe. At maximum there could be 1% of the hydrogen mass hidden in these small bodies which is not enough to explain dark matter. The collection of small and low mass objects are also called MACHOs, Massive Astrophysical Compact Halo Object. Tisserand et al. (2007) ruled out MA- −7 CHOs in the mass range of 0:6 10 M < M < 15M to be the primary occupants of the Milky Way Halo.× They tried to detect the MACHOs by mon- itoring the change of brightness of many stars in the Magellanic Clouds when a MACHO in the Milky Way Halo moves in front of the background star. Instead of the 39 predicted MACHO events they found only one candidate. Massive stellar black holes formed during the collapse of massive stars (M∗ . 100M ) pollute their environment by expelling parts of their matter before collapsing into a black hole. The abundance of heavy elements in the expelled matter puts constraints on the number of massive blackholes which can be formed without interfering with the universal mass density. Carr et al. −4 (1984) showed that ΩBH . 10 , where ΩBH is the contribution of massive black holes to the universal mass density. For stars much heavier than M∗ & 100M , there is no such constraint since they collapse without polluting their environment (Carr et al., 1984). But there is an upper limit on the mass of the black holes due to the constraints from the ve- locity dispersion-age relation in the solar neighbourhood. Another constraint is the absence of lensing effects in most quasars, making the cosmological number density of massive black holes formed from massive stars negligible (Canizares, 1982). 2.1.2 Non-baryonic dark matter Non-baryonic dark matter is matter consisting of non-baryonic particles. Many candidate particles are predicted by extentions to the standard model but only one dark matter candidate has been detected so far. Neutrinos were postulated by Wolfgang Pauli in 1930 to explain the conser- vation of energy and momentum in beta decay. In 1995 the Nobel prize was awarded to Frederick Reines for the detection of the free neutrino in experiments carried out in 1956. The neutrino has no charge, half integer spin and comes in three flavours: e-, µ- and τ-neutrino. They are formed by beta decay of three other leptons, the electron, muon and tau lepton respectively. In 1998 experi- ments have shown that neutrinos possess mass, since it is possible for them to oscillate from one flavour to another. From Solar neutrinos it is known that they can change shape between being emitted from the Sun and observed at Earth.
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